1. Field of the Invention
The present invention relates to a process for preparing halogenated polymers and to a device for its implementation.
2. Description of the Related Art
Processes for the preparation of halogenated polymers in a medium comprising liquid or supercritical carbon dioxide generally may require high working pressures, in order to ensure the temperature and the density that are necessary for polymerization reactions. These process may also generally require low degrees of conversion, in order to ensure a moderate solids content guaranteeing free flow of the polymer suspension towards the outlet of the reactor. The latter requirement may be incorporated because of the high porosity of crystalline or semicrystalline polymers, which often results from their precipitation under conditions of very low solubility in liquid or supercritical carbon dioxide.
Various processes for preparing halogenated polymers and, in particular, fluoropolymers in liquid or supercritical carbon dioxide have been proposed. For instance, U.S. Pat. No. 5,674,957 describes the preparation of fluoropolymers by the polymerization of fluoromonomers in carbon dioxide in the presence of initiators capable of forming stable chain ends on the fluoropolymers. This polymerization is optionally followed by a step of separating the polymer formed, involving the complete decompression to atmospheric pressure of the polymerization medium and the recovery of the polymer by physical separation. Optionally, the polymer is washed before being processed.
WO 98/28351 describes a process for polymerizing fluoromonomers in a medium comprising liquid or supercritical carbon dioxide in a pressurized reactor and the recovery of the fluoropolymer by flash decompression of the polymerization medium extracted from said reactor. After having been recovered, the polymer obtained is optionally subjected to a subsequent heat treatment before processing, in order to remove therefrom the potentially gas-generating compounds.
One of the drawbacks of these processes is that all of the carbon dioxide and of the unconverted monomers accompanying the polymer to the outlet of the reactor is expanded, either by flash decompression or by complete decompression to atmospheric pressure, and therefore that the recycled fraction of this carbon dioxide and of these unconverted monomers must be returned to the polymerization pressure by compression and/or by condensation and pumping, these operations penalizing the cost-effectiveness of the process because of their energy cost. Another drawback of these processes is that they do not provide for the purification of the polymers during their formation and it is then necessary to subject them to a treatment before they are processed.
WO 01/34667 presents a continuous process for polymerization in a medium comprising liquid or supercritical carbon dioxide in a pressurized reactor, with separation of the carbon dioxide and unconverted monomers of the polymer at a pressure close to the reaction pressure, the solid polymer being retained in the separator while the carbon dioxide and the unconverted monomers are recycled back to the reactor. In the preferred device for separating the polymer, this operation is carried out by accumulation of the solid polymer in filters operating in alternating accumulation/discharge cycles.
One of the drawbacks of this process is the difficulty of dislodging the solid polymer from the filter where it has accumulated, with the consequent risks of total or complete blockage, long residence time of certain polymer fractions, perturbations in the polymerization and the separation conditions, which may lead to certain fractions of the production being outside the quality standard. Another drawback of this process is the cost of the separation equipment and of the equipment for controlling the accumulation/discharge cycles needed to separate the solid particles.